1. Field of the Invention
The present invention relates to a surface acoustic wave filter utilizing surface acoustic waves.
2. Description of the Prior Art
With the recent rapid spread of automobile telephones and portable telephones, the necessity for surface acoustic wave high-frequency filters, is increased as a small-sized and high-performance high-frequency filter. A transversal type filter which is one type of the surface acoustic wave high-frequency filters has been already put to practical use.
As a further subject of improving characteristics, however, it is desired to reduce the loss so that no matching circuit is required. Therefore, a resonator type filter is paid attention to (see Japanese Patent Publication No. 19765/1981). In addition, an interdigitated interdigital transducer type surface acoustic wave filter is similarly paid attention to.
Various methods of constructing resonator type filters have been considered. Description is made of a ladder type surface acoustic wave high-frequency filter which is one of the resonator type filters.
The surface acoustic wave high-frequency filter is constructed by connecting in series one or more filter functional units each comprising a surface acoustic wave resonator. FIG. 21 is a circuit diagram showing a filter functional unit. The filter functional unit comprises a first surface acoustic wave resonator 343 having input and output terminals connected in series with a signal line and a second surface acoustic wave resonator 344 having input and output terminals one of which is connected to the signal line and the other of which is grounded. Each of the resonators 343 and 344 has double resonance characteristics. In addition, reference numerals 345 and 346 in FIG. 21 denote input and output terminals in the signal line, and 347 denotes the ground.
FIG. 22 is a graph showing the idealized impedance characteristics of the surface acoustic wave resonator. For simplicity of illustration, impedance shall be pure reactance. The impedance of the resonator 343 connected in series with the signal line is indicated by X.sub.1 and the impedance of the resonator 344 connected in parallel with the signal line is indicated by X.sub.2.
Since each of the resonators has double resonance characteristics, it has two resonance frequencies whose impedance is zero and infinity. The frequency whose impedance is zero is referred to as a resonance frequency or a resonance point, and the frequency whose impedance is infinity is referred to as an antiresonance frequency or an antiresonance point.
If the resonance point of the resonator 343 and the antiresonance point of the resonator 344 coincide with each other, signals are passed in the vicinity of the frequency because the resonator 343 enters an ON state and the resonator 344 enter an OFF state. On the other hand, the resonator 343 enters an OFF state at the antiresonance point, whereby an attenuation pole is produced on the side of frequencies higher than those in the passband. In addition, the resonator 344 enters an ON state at the resonance point, whereby an attenuation pole is produced even on the side of frequencies lower than those in the passband.
Several filter functional units are connected in series, thereby to obtain characteristics required for a band-pass filter. FIG. 29 is a schematic plan view showing a surface acoustic wave high-frequency filter in which three filter functional units are connected in series, and FIG. 30 is a diagram showing its equivalent circuit. Each of surface acoustic wave resonators 407 to 409 is connected in parallel with a signal line, and each of surface acoustic wave resonators 410 to 412 is connected in series with a signal line. A first filter functional unit is constituted by the surface acoustic wave resonators 407 and 410, a second filter functional unit is constituted by the surface acoustic wave resonators 408 and 411, and a third filter functional unit is constituted by the surface acoustic wave resonators 409 and 412. The surface acoustic wave resonators 407 to 412 are two-terminal resonators having double resonance characteristics. In addition, the surface acoustic wave high-frequency filter has input and output terminals 450 and 451.
The problem is that the actual surface acoustic wave resonator does not exhibit ideal impedance characteristics as shown in FIG. 22.
The problem will be considered in terms of the characteristics of reflectors. For example, a resonator comprising an interdigital transducer 348 and electrically open strip type grating reflectors 349 and 350 as shown in FIG. 23 has impedance characteristics as shown in FIG. 24.
Furthermore, a resonator comprising an interdigital transducer 351 and electrically short strip type grating reflectors 352 and 353 as shown in FIG. 25 has impedance characteristics as shown in FIG. 26. In FIGS. 24 and 26, a solid line indicates a real resistance component, and a dotted line indicates a reactance component.
The characteristics of the resonator shown in FIG. 23 are distorted in the vicinity of the antiresonance point. The characteristics of a surface acoustic wave high-frequency filter in which filter functional units in three stages each comprising the resonator are connected are as shown in FIG. 27. As can be seen from FIG. 27, both the in-band insertion loss and the out-of-band attenuation cannot be satisfied.
On the other hand, the characteristics of the resonator shown in FIG. 25 are distorted in the vicinity of the resonance point. The characteristics of a surface acoustic wave high-frequency filter in which filter functional units in three stages each comprising the resonator are connected are as shown in FIG. 28, which cannot be also satisfied, although they are improved over the characteristics shown in FIG. 27.
Furthermore, in the surface acoustic wave filter, it is not generally easy to ensure high attenuation outside the band irrespective of the characteristics of the reflectors, as apparent from frequency characteristics shown FIG. 31. Specifically, there is a problem of antimony that an attempt to obtain high attenuation outside the band increases the insertion loss.
As an effective solution for the problem, the impedance characteristics of the resonator connected in series with the signal line are made steeper while holding the frequency difference between the resonance point and the antiresonance point, as compared with the impedance characteristics of the resonator connected in parallel with the signal line.
Examples of a method of making the impedance characteristics steep include a method of decreasing the number of electrode fingers in the interdigital transducer. In this method, however, the impedance is changed asymmetrically on the side of low frequencies and high-frequencies, and the frequency difference between the resonance point and the antiresonance point is changed. Accordingly, this method is not suitable for the above described object.
Furthermore, there is a method of decreasing the cross width of electrode fingers (see Japanese Patent Laid-Open Gazette No. 81823/1992). If the cross width of electrode fingers is decreased to approximately a fraction of the original one, factors which are difficult to calculate such as the stray capacitance on end surfaces of the electrode fingers or an interaction between surface acoustic waves and a connecting electrode between the electrode fingers cannot be ignored. Accordingly, the frequency difference between the resonance point and the antiresonance point is changed, thereby to make it difficult to obtain high attenuation outside the band.
Description is now made of an interdigitated interdigital transducer type surface acoustic wave filter. FIGS. 35 and 36 illustrate conventional interdigitated interdigital transducer type surface acoustic wave filters. Two or more comb-shaped input electrodes 501a are formed in an input portion 501, and one or more comb-shaped output electrodes 502a are formed in an output portion 502. The comb-shaped input and output electrodes 501a and 502a are alternately disposed on the same propagation path.
A pair of strip type reflectors 503 and 503' or reflectors 504 and 504' is provided on the outermost side of a group of the comb-shaped input and output electrodes for the purpose of reducing the insertion loss. The reflectors 503 and 503' are referred to as ones in a comb shape (of an open strip type), in which reflector elements 503b . . . (503b'. . . ) extending from conductor portions 503a and 503a (503a' and 503a') oppositely arranged which constitute the reflector 503 (503') are so formed as not to be brought into contact with the opposite conductor portion 503a (503a'). On the other hand, the reflectors 504 and 504' are referred to as grating ones (of a short strip type), in which reflector elements 504b . . . (504b'. . . ) extending from conductor portions 504a and 504a (504a' and 504a') oppositely arranged which constitute the reflector 504 (504') are so formed as to be brought into contact with the opposite conductor portions 504a (504a').
The conventional interdigitated interdigital transducer type surface acoustic wave filters are respectively so constructed that the respective one conductor portions 503a and 503a' opposite to each other in both the reflectors 503 and 503' are connected to the ground, and the respective one conductor portions 504a and 504a' opposite to each other in both the reflectors 504 and 504' are connected to the ground.
The surface acoustic wave filter comprising the grating reflectors 504 and 504' out of the interdigitated interdigital transducer type surface acoustic filters of both the constructions has the disadvantage in that a ripple exists in the frequency characteristics. On the other hand, it is known that the conventional surface acoustic wave filter comprising the comb-shaped reflectors 503 and 503' allows a ripple produced by the effect of the reflectors to be reduced to some extent. Even in the interdigitated interdigital transducer type surface acoustic wave filter using the comb-shaped reflectors 503 and 503', however, a ripple of approximately 1 dB is produced within the passband, which does not satisfy the stability as a high-frequency element.